Not 100% sure if this should go in bug or ux, my apologies if I post in the wrong place.
I opened this link New search operator "not" for tags only - #2 by jomaxro and noticed that a broken thumbnail can cause an annoying issue (tested on Chrome and Edge). A large, broken image. Also you can’t select text or select links under it.
The issue doesn’t show up if I create a new onebox of the same link since the generated thumbnail has a different, working url.
ah ok I see, that’s from the topic New search operator "not"
It looks like it might have something to do with our lazyloaded images… it has the correct positioning applied inline (looks like this is all from lazy-load-images.js.es6),
position: absolute;
top: 82px;
left: 28px;
but the width and height are out of whack at
width: 500px;
height: 500px;
I just tried the “rebuild html” option on that post and that corrected it. That post predated our lazyload feature, so I wonder if it was just a fluke when we rebuilt all the posts after that feature was added?
Oh I spoke too soon, now it’s broken in a different way… and it just fixed itself again. Something a bit odd is going on… if I do a hard refresh I will temporarily see the huge image…
@dan it looks like you recently made some changes there… could this possibly be related?
Trying another onebox…
and this is what it looks like in the composer preview
Thanks @Canapin for the report. It is a bug I introduced by mistake after fixing another bug. 
It is now fixed:
https://github.com/discourse/discourse/commit/2d3113e4da74be2a0288dbe3273093cd2d27fd21
